The present invention relates to the general field of nozzles that are fitted to airplane turbojets. The invention relates more particularly to the separate-stream nozzles fitted with stream mixers that served to improve the performance of the engine while also reducing the noise emitted by the jet leaving the outlet from the nozzle.
More precisely, as shown very diagrammatically in FIG. 1, a separate-stream nozzle for a turbojet typically comprises, from the inside towards the outside, a central body 12 (also referred to as a “plug”), an inner shroud 14, a secondary cap or outer cowling 16, and a nacelle 19 centered on the axis X-X′ of the nozzle.
The inner shroud 14, of substantially cylindrical shape, extends along the axis X-X′ of the nozzle, the central body 12 being placed concentrically inside the outer shroud 14 and terminating in a portion that is substantially conical. The inner shroud 14 defines a first annular channel for passing a hot inner stream (or primary stream) coming from the combustion chamber. The inner shroud corresponds to the exhaust casing of the nozzle.
The secondary cap 16 is disposed concentrically around the inner shroud 14 and it co-operates with the nacelle 19 to define a second annular channel for passing a cold outer stream (or secondary stream) coming from the fan.
In known manner and in order to reduce specific consumption, the nozzle is provided with a mixer 18 having a special design to encourage mixing between the hot inner stream flint and in the cold outer stream flext coming from the turbojet. As shown in FIG. 1, the mixer 18 comprises a lobed structure 20 that represents one of the designs that is presently in the most widespread use in civil turbojets. The lobed mixer serves to obtain radial shear between the hot inner stream and the cold outer stream so as to encourage mixing between these streams. The mixer 18 is fastened to the inner shroud 14.
Embodiments of lobed mixers for separate-stream nozzles are described in particular in the following documents: EP 1 141 534, U.S. Pat. No. 5,755,092, and U.S. Pat. No. 5,924,632.
Nevertheless, although a mixer of that type does indeed improve the efficiency and the noise performance of a turbojet having a separate-stream nozzle, it inevitably leads to an increase in the weight of the nozzle, with that having an impact on the overall dynamics of the engine and on its connection with the pylon of the airplane.
In the technique that is the most widespread at present, the lobed mixer is made as a single piece using a metallic material, typically Inconel® 625, the lobed structure being connected to the inner shroud via a Y-shaped ring enabling an outer cowling also to be fastened. Such a mixer presents non-negligible additional weight that is cantilevered out in the engine, thereby leading to an increase in the mechanical loading on the flange of the exhaust casing of the nozzle. Furthermore, the Y-shaped ring is heavily stressed by the temperature gradients present between the first annular channel for passing the hot inner stream and the second annular channel for passing the cold outer stream.
In order to reduce the mechanical loads generated by the presence of such a weight at the outlet from the nozzle, one solution consists in making the major fraction of the surface of the mixer, i.e. the lobed structure, out of a ceramic matrix composite material (CMC material), which material is lighter than a metallic material.
FIG. 2 shows a mixer 40 that comprises a lobed structure 41 made of CMC, and a fastener shroud 42 made of metallic material for connecting the mixer to the exhaust casing of a nozzle. The lobed structure 41 also includes a stiffener ring 43 interconnecting the inner lobes of the structure in order to reinforce the mechanical strength of the lobed structure overall. That mixer is described in detail in the document WO 2006/035186.
The operation of that mixer has been proven, but although its architecture enables a significant weight saving to be achieved (about 40%) compared with a one-piece metallic mixer, it nevertheless still presents certain drawbacks.
Incorporating the lobed structure 41 made of CMC material requires a large number of metallic parts to be used, thereby encroaching on the weight savings. In particular, as shown in FIG. 3, the lobed structure 41 is fastened to the inner shroud 422 of the fastener shroud 42 via flexible metallic fastener tabs 43 that serve to compensate for differential expansion between the lobed structure made of CMC material and the inner shroud made of metallic material. Likewise, in order to accommodate differential expansion between the outer shroud 421 and the inner shroud 422, which are subjected to streams at different respective temperatures (cold outer stream and a hot inner stream), flexible metallic fastener tabs 44 are used to fasten the inner shroud 422 to the outer shroud 421.
Furthermore, those flexible connections, and in particular the tabs 43, need to accommodate steep temperature gradients leading to high levels of mechanical stress in the lobed structure, thereby reducing its lifetime.
Finally, the presence of the stiffener ring gives rise to losses of performance, leading to an increase in the specific consumption of the turbojet, thereby limiting to some extent the benefit obtained by lightning the weight of the mixer.